Shocking recipe for making killer electrons

Artist's impression of the Cluster constellation.
ESA's mission Cluster consists of four identical spacecraft flying in formation between 11,800 miles (19, 000 kilometers) and 74,000 miles (119,000 kilometers) above the Earth. They study the interaction between the solar wind and Earth's magnetosphere, or the Sun-Earth connection in 3D.

ESA

March 12, 2010Take a bunch of fast-moving electrons, place them in orbit and then hit them with the shock waves from a solar storm. What do you get? Killer electrons. That's the shocking recipe revealed by the European Space Agency's (ESA) Cluster mission.

Killer electrons are highly energetic particles trapped in Earth's outer radiation belt, which extends from 7,500 miles (12,000 kilometers) to 39,800 miles (64,000 kilometers) above the planet's surface. During solar storms their number grows at least ten times, and they can be dislodged, posing a threat to satellites. As the name suggests, killer electrons are energetic enough to penetrate satellite shielding and cause microscopic lightning strikes. If these electrical discharges take place in vital components, the satellite can be damaged or even rendered inoperable.

On November 7, 2004, the Sun blasted a solar storm in Earth's direction. It was composed of an interplanetary shock wave followed by a large magnetic cloud. When the shock wave first swept over the ESA-NASA solar watchdog satellite SOHO, the speed of the solar wind (the constant flow of solar particles) suddenly increased from 300 miles (500 kilometers) per second to 400 miles (700 kilometers) per seconds.

Shortly afterwards, the shock wave hit Earth's protective magnetic bubble, known as the magnetosphere. The impact induced a wave front propagating inside the magnetosphere at more than 700 miles (1,200 kilometers) per second at geostationary orbit (22,000 miles [36,000 kilometers] altitude) around Earth. The quantity of energetic electrons in the outer radiation belt started to increase too, according to Cluster's Research with Adaptive Particle Imaging Detectors (RAPID). Cluster's four satellites sweep around an elliptical orbit, coming as close as 12,000 miles (19,000 kilometers) and going out as far as 74,000 miles (119,000 kilometers).

Understanding the origin of the killer electrons has been a focus for space weather researchers. Thanks to previous data collected by Cluster and other space missions, scientists proposed two methods by which electrons can be accelerated to such harmful energy levels. One relies on very low frequency (VLF) waves of 3-30 kHz, the other on ultra low frequency (ULF) waves of 0.001-1 Hz. This latest work disentangles the problem.

Which waves are responsible? Both of them. "Both VLF and ULF waves accelerate electrons in Earth's radiation belts, but with different timescales. The ULF waves are much faster than the VLF, due to their much larger amplitudes," said Qiugang Zong, from Peking University in China and University of Massachusetts Lowell.

The data show that a two-step process causes the substantial rise of killer electrons. The initial acceleration is due to the strong shock-related magnetic field compression. Immediately after the impact of the interplanetary shock, Earth's magnetic field lines began wobbling at ultra low frequencies. In turn, these ULF waves were found to effectively accelerate the seed electrons provided by the first step to become killer electrons.

Although the analysis has been a long one, the results have been worth the wait. Now astronomers know how killer electrons are accelerated. "Data from the four Cluster satellites allowed the identification of ULF waves able to accelerate electrons," said Malcolm Dunlop, Rutherford Appleton Laboratory, United Kingdom.

Thanks to this analysis of Cluster data, if the killer electrons happen to be ejected towards Earth, we now know that they can strike the atmosphere within just 15 minutes. "These new findings help us to improve the models predicting the radiation environment in which satellites and astronauts operate. With solar activity now ramping up, we expect more of these shocks to impact our magnetosphere over the months and years to come," said Philippe Escoubet, ESA's Cluster mission manager.